Process and apparatus for multilayer blow molding and articles blow-molded therewith

ABSTRACT

This invention provides a blow molding process for manufacturing multilayer hollow articles having sections different from one another in respect to the kind of resins, number of layers, and thickness of layers along the circumference in the horizontal section of an article with the narrowest of the sections in any one of the resin layers extending in a specified width along the longitudinal wall of an article and being capable of satisfying the sectional performance requirements, an apparatus, and said hollow articles. It also provides a process for manufacturing multilayer hollow articles with the wall thickness ratio of resin layers in each section varying along the longitudinal wall of an article or those with the sectional width in each resin layer varying along the longitudinal wall of an article, an apparatus, and said hollow articles. Hollow articles molded by the process and apparatus of this invention are particularly useful for automotive bumpers, the seats and backs of chairs, and boiler-room doors.

This is a division of application Ser. No. 07/995,261, filed Dec. 22,1992, now U.S. Pat. No. 5,460,772.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

This invention relates to a novel process for multilayer blow molding,an apparatus to practice the process, and hollow articles molded by theprocess. More particularly, it relates to a process for multilayer blowmolding to produce hollow articles which have two or more layers in thetransverse direction and also two or more sections of different resinsin any one of the multiple layers around the circumference, to amultilayer blow molding apparatus to practice the process, and to hollowarticles molded by the process.

Numerous proposals have been made on blow molding processes forproducing hollow articles having multiple layers of several kinds ofthermoplastic resins, apparatuses, or hollow articles. Some of them arecited below by the structure multilayer parisons to be created.

A multilayer parison P disclosed in Japan Kokoku Tokkyo Koho Nos. Sho52-37,026 (1977) and Sho 57-53,175 (1982) and others has resin layers 1and 2 concentrically formed by resins A and B in its horizontal crosssection with the ratio of the thickness of resin layer 1 to that ofresin layer 2 maintained constant in every part, namely, in thelongitudinal direction (the direction of extrusion) and in thecircumferential direction as illustrated, for example, in FIG. 49. Blowmolding of such multilayer parison P is practiced commercially toproduce fuel tanks for automobiles and mayonnaise and ketchup containersfor home consumption.

A process known as modified multilayer blow molding disclosed in JapanKokai Tokkyo Koho Nos. Sho 62-138,227 (1987) and Hei 2-113,908 (1990)and others creates a multilayer parison P, such as illustrated in FIG.50, which has annular resin layers 1 and 2 concentrically formed byresins A and C as in FIG. 49 cited above and, in addition, has a resinlayer 3 of another resin B between the resin layers 1 and 2 in aspecified region in the longitudinal wall of the parison. The blowmolding of automotive fuel tanks is described here as main applicationof such multilayer parison P.

A process known as connection blow molding disclosed in Japan KokaiJitsuyo Shinan Koho Nos. Sho 63-101,512 (1988) and Sho 63-106,984 (1988)creates a multilayer parison P, such as illustrated in FIG. 51, whichhas annular resin layers 1 and 2 concentrically formed by resins A and Bas in FIG. 49 cited above with the ratio of the thickness of resin layer1 to that of resin layer 2 varying in the longitudinal direction. Theproduction of air intake ducts to be installed in automotive enginerooms is described here as main application of the blow molding of suchmultilayer parison P. In FIG. 51, the resin layer 1 is thicker than theresin layer 2 at both upper and lower ends of the parsion while theresin layer 2 is thicker than the resin layer 1 in the middle region,but the pattern of the change in the ratio of wall thickness of resinlayers is not limited to the one described here.

A process known as exchange blow molding described in Japan KokokuTokkyo Koho No. Hei 2-15,373 (1990) discloses a multilayer parison P,such as illustrated in FIG. 52, which has parts of resin A, resin B, andresin A in succession from one end to the other in the longitudinaldirection for molding air intake ducts to be installed in automotiveengine rooms.

Furthermore, a multilayer parison disclosed in Japan Kokai JitsuyoShinan Koho No. Hei 3-57,020 (1991), an invention by one of the presentinventors, has two semicircular sections in its horizontal crosssection, one section having a two-layer structure consisting of alaminate of two kinds of resins and another single-layer structure ofone kind of resin, with this layered structure maintained in thelongitudinal direction from one end to the other. The parison inquestion is described to be advantageous for molding containers fortransporting a variety of goods. However, a parison of this layeredstructure warps in wrinkles towards the single-layer side as soon as itemerges from a die head as shown in FIG. 29 and cannot possibly be usedadequately in molding. Besides, the process here is an extremelysingular one which utilizes a seven-part mold and it is limited to themolding of special articles of a deep-drawn double-wall structure, notapplicable to articles of other shapes. Moreover, no mention is made ofa concrete method and a mechanism for molding this type of multilayerparisons in Japan Kokai Jitsuyo Shinan Koho No. Hei 3-57,020 (1991).

The properties of hollow articles produced by blow molding dependprimarily on those of thermoplastic resins in use. Therefore, in orderto improve the properties of hollow articles, it is extremely importantfor the articles to vary in properties from region to region and satisfyperformance requirements for each region in particular end uses andplaces of usage.

In automotive bumpers, for example, the fascia is required to bebeautiful to look and the beam strong to withstand the shock from acollision. In the seat and back of a chair, the face side that comesinto direct contact with a person is required to be made of materialswhich are elastic, soft to the touch, and not slippery. On the otherhand, the back side which is fixed to a frame such as legs to supportthe weight of a person must have excellent strength and toughness. Inthe case of a boiler-room door, its outer side is required to have goodappearance as it is exposed to human eyes while its inner side providesa barrier against an atmosphere containing high-temperature steam andvapor and floating oil drops and hence must be resistant to heat, hotwater, and oils. In addition, the upper and lower edges of the door areplaced in contact with lintels and thresholds and move on rails when thedoor is opened or closed and hence they are required to be abrasion- andheat-resistant. In the production of the above-mentioned articles, it isdesirable to select resins which provide the properties for particularperformances of various parts of the articles and mold the selectedresins into integrated articles.

In the blow molding of the above-mentioned articles, the performancerequirements may not be satisfied by providing a multilayer structuremerely in the transverse direction and the creation of multiple sectionsof different kinds of resins becomes essential in the circumferentialdirection where multilayer parisons are utilized. Also, it becomesnecessary to create a multilayer structure in the transverse directionof a parison and multiple sections of different kinds of resins in thecircumferentical direction of a parison.

Although the above-mentioned conventional multilayer blow moldingprocesses are able to create a multilayer structure in the transversedirection of a parison, they are unable to create sections of differentkinds of resins in the circumferential direction of a parison. Inconsequence, they are unable to mold hollow articles which can satisfylocally different performance requirements from plural resins havingappropriate properties.

Such being the case, the only way to manufacture, for example,automotive bumpers by a conventional process was to mold a fascia and abeam separately and there was no way to mold the two as an integratedarticle in order to reduce the production cost. Likewise, in themanufacture of the seat or the back of a chair, the face side and theback side had to be molded separately and put together later.Unavailability of a single-step process for the manufacture of anintegrated article resulted in higher production cost and causedproblems in strength.

OBJECT AND SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a process forblow molding hollow multilayer articles which have sections differentfrom one another in respect to one or more of the kind of resins, numberof layers, and thickness of layers along the circumference of the crosssection, one with the smallest width of said sections of different kindsof resins in any one of the resin layers having a width of 1/8 or moreof the circumference and extending along the longitudinal wall of themolded article and each section satisfying its respective performancerequirements and also provide an apparatus and hollow articles.

Another object of this invention is to provide a process for blowmolding hollow multilayer articles which have sections different fromone another in respect to one or more of the kind of resins, number oflayers, and thickness of layers along the circumference of the crosssection, the ratio of the wall thickness of resin layers in each sectionvarying along the longitudinal wall of the molded article and thecreation of said sections and the changes in the ratio of wall thicknessalong the longitudinal wall being effected to satisfy locally differentperformance requirements of the article and also provide an apparatusand hollow articles.

A further object of this invention is to provide a process for blowmolding hollow multilayer articles which have sections different fromone another in respect to one or more of the kind of resins, number oflayers, and thickness of layers along the circumference of the crosssection, the width of each section created in each resin layer varyingalong the longitudinal wall of the molded article and each sectionsatisfying its respective performance requirements and also provide anapparatus and hollow articles.

A still further object of this invention is to provide blow-moldedhollow multilayer articles, for example, automotive bumpers, the seatsand backs of chairs, and boiler-room doors, which have multiple sectionsof different kinds of resins with each section satisfying its respectiveperformance requirements.

Accordingly, the first mode of the process of this invention relates toa multilayer blow molding process for extruding plural thermoplasticresins from a die head in a tubular form to create a parison which ismultilayer all around the whole circumference and substantially uniformin wall thickness in every part, introducing said multilayer parisoninto a mold which is split into several parts and open to receive saidparison, and clamping said mold to conduct blow molding which comprisescreating a multilayer parison having sections different from one anotherin respect to one or more of the kind of resins, number of layers, andthickness of layers along the circumference of said parison, with one ofthe smallest width of said sections of different kinds of resins in anyone of resin layers extending in the longitudinal direction of saidparison in a sectional width corresponding to a central angle of 45° ormore.

In this process, the outermost layer of a multilayer parison is the onewhich essentially demarcates sections of different kinds of resins andthe mold is divided into a number of parts corresponding to saidsections of different kinds of resins. When the mold is clamped toperform blow molding, these parts while meeting with one another cut offthe boundaries between the sections of different resins and match theboundaries of the sections of different resins along the edge of asquare part of the molded article, thus avoiding with certainty themismatch of said boundaries of the sections of different resins and thedamages to the aesthetic quality of the molded article.

An apparatus to practice the first mode of the process of this inventionextrudes plural thermoplastic resins from a die head in a tubular formto create a parsion which is multilayer all around the circumference andsubstantially uniform in thickness in every part, introduces saidmultilayer parison into a mold which is split into plural parts and opento receive said parison, and clamps said mold to perform blow moldingand it may be constructed in the following manner. The die head inquestion is composed of a multitorus which spreads molten masses ofplural resins emerging from extruders into a concentric annular passage,a lotus root which provides passages for the resins to create sectionsof different resins in each resin layer in the circumferential directionof the parison, an octopus which forms passages between the multitorusand the lotus root, and a nozzle which is located beneath the lotus rootand extrudes a multilayer parison. Designating the number of kinds ofresins as m (m is an integer and ≧2), that of layers in the extrudedparison as n (n is an integer and ≧2), and that of sections differentfrom one another in respect to one or more of the kind of resins, numberof layers, and thickness of layers to be formed in the circumferentialdirection of the parison as p (p is an integer and 8≧p≧2), an apparatusmay be constructed so that the multitorus has passages to spread moltenresins emerging from m inlets into m-fold concentric annular passage,the lotus root has passages to distribute the molten resins to form anm-resin-n-layer-p-section pattern, and the octopus has passages todirect the molten resins previously spread into an m-fold concentricannular passage in the multitorus towards the lotus root having thepassages for the m-resin-n-layer-p-section pattern.

In the multilayer blow molding apparatus mentioned above, themultitorus, lotus root, octopus, and nozzle constituting the die headare made independently for easy disassembly and reassembly. This makesit possible to choose a variety of configurations for the lotus root orto choose the kind and number of molten resins to be fed to themultitorus to create multilayer parisons of a variety of structures inrespect to the number of kinds of resins m, number of layers n, andnumber of sections p, and hollow articles molded from such parisons canadequately meet the needs for the articles.

With the use of this multilayer blow molding machine, the attachment ofdetachable blind rings to the lower openings of the passages in thelotus root to block the outer (n-n') layers out of the passages for thewhole n layers makes it possible to utilize a die head designed for theextrusion of multilayer parisons of an m-resin-n-layer-p-section pattern(n is an integer and ≧3) for the creation of multilayer parisons of anm'-resin-n'-layer-p'-section pattern (m', n', and p' are respectively 2or more and m≧m', n>n, and p≧p'). Thus, a die head with a givencombination of the multitorus, lotus root, octopus, and nozzle cancreate multilayer parisons having a smaller number of layers and thus amultipurpose apparatus can be constructed.

With the use of a multilayer blow molding apparatus constructed asabove, plural thermoplastic resins molten separately are broughttogether in the die head and extruded through a slit shaped like acircle, an ellipse, or a combination of a circular arc and a straightline in the nozzle located at the tip of the die head to createmultilayer parisons having a variety of m-resin-n-layer-p-sectionpatterns and said parisons are molded to yield hollow articles havingplural sections of different kinds of resins with each sectionsatisfying locally different performance requirements.

Accordingly, the hollow multilayer articles produced by the first modeof the process and apparatus relate to hollow articles molded byextruding plural thermoplastic resins from a die head in a tubular formto create a parsion which is multilayer all around the circumference andsubstantially uniform in thickness in every part and blow molding saidmultilayer parison. The hollow articles have sections different from oneanother in respect to the kind of resins, number of layers, andthickness of layers in the circumferential direction of the crosssection with a section of the smallest width of said sections ofdifferent kinds of resins in any one of the resin layers extending alongthe longitudinal wall of the article in a width of 1/8 or more of thecircumference and each section satisfying its respective performancerequirements.

An example of the multilayer articles blow-molded by the first mode ofthe process and apparatus of this invention is automotive bumpers. It isrecommended here to choose ABS resin for the fascia for its goodappearance and glass fiber-reinforced polypropylene for the beam for itsstrength to withstand the shock from a collison and for its relativelylow cost. Another example is boiler-room doors. It is recommended tochoose ABS resin for the outer side for its good appearance, glassfiber-reinforced polyphenylenesulfide for the inner side for itsexcellent resistance to heat, oils, and hot water, and polyoxymethylenefor the upper and lower edges for its excellent abrasion and heatresistance.

The second mode of the process of this invention relates to a multilayerblow molding process for extruding m kinds (m is an integer and ≧2) ofthermoplastic resins from a die head in a tubular form to create amultilayer parison, introducing said parison into a mold which is splitinto plural parts and open to receive said parison, and clamping saidmold to perform blow molding which comprises creating a multilayerparison having p sections (p is an integer and ≧2) of different kinds ofresins extending in the longitudinal direction of the parison in a widthof a given central angle in the circumferential direction and n layers(n is an integer of 1 or more and is 2 or more in one or more sections)in each section, and making the wall thickness of said multilayerparison substantially uniform regardless of the number of layers n ineach section by providing n_(k) passages (1≦k≦p) for molten resins inthe die head and feeding any one of the resins constituting the sectionsother than the kth section through additional passages or a total ofn_(k) passages to said other sections where the number of layers in thefirst section is designated as n₁, that in the second section as n₂,that in the pth section as n_(p), and that in the kth section which isthe largest as n_(k).

An example of multilayer blow molding disclosed in Japan Kokai JitsuyoShinan Koho No. Hei 3-57,020 (1991) relates to the formation of amultilayer parison which has two equal sections (p=2) with a sectionalwidth of 180°, one section being composed of multiple layers of resinsand the other a single layer of resin. The experiments by the presentinventors indicate, however, that the flow rate of molten resins througha multilayer section becomes greater than that through a single-layersection and this tends to cause the multilayer parison being formed towarp in wrinkles towards the side of the smaller flow rate, often makingthe molding impossible. A remedial procedure in such a case is to letmolten resins flow through the same number of passages in thesingle-layer section as in the multilayer section so that an apparentsingle layer is actually composed of multiple layers of the same resin.In this way, it is possible to make the wall thickness of a multilayerparison being created substantially uniform and maintain a balance ofthe flows of molten resins through the multilayer section and theapparent single-layer section. The experiments of the present inventorsindicate that it is advisable to control the total flow of molten resinsthrough the multilayer section at 0.7 to 1.3 times, preferably 0.8 to1.2 times, more preferably 0.9 to 1.1 times, the flow of molten resinthrough the apparent single-layer section. Such control of the flow ofmolten resins enables the creation of a multilayer parison with asubstantially uniform thickness in every part regardless of thedifference in the number of layers n in each section.

A multilayer blow molding apparatus to practice the second mode of theprocess of this invention extrudes plural thermoplastic resins from adie head in a tubular form to create a multilayer parison with asubstantially uniform thickness in every part, introduces said parisoninto a mold which is split into plural parts and open to receive saidparison, and clamps said mold to perform blow molding. Designating thenumber of kinds of resins to be used for the extrusion of a multilayerparison as m (m is an integer and ≧2), the number of layers in theextruded multilayer parison as n (n is an integer equal to 1 or more andis 2 or more in one or more sections), and the number of sectionsdifferent from one another in respect to one or more of the kind ofresins, number of layers, and thickness of layers in the circumferentialdirection of the parison as p (p is an integer and 8≧p≧2), the aforesaiddie head is composed of a multiple annular passage which is constructedby assembling passage-forming tubes in the number corresponding to n anda shell and a nozzle which is attached to the lower end of the multipleannular passage and extrudes a multilayer parison. The aforesaidpassage-forming tubes have slots in the outer wall in the numbercorresponding to p and form passages for molten resins coupled with theinner wall of the adjacent outside tubes and resin distribution tubesconnect the tips of extruders in the number corresponding to that of thekind of resins m and the upper end of each passage in the aforesaidmultiple annular passage.

In this multilayer blow molding apparatus, it is desirable to providethe same number of extruders as that of the number of kind of resins m,at least one branch-off point and at least one junction in a resindistribution tube and a flow switchover valve at the branch-off point orthe junction or both. The provision of a branch-off point and a junctionin a resin distribution tube and a flow switchover valve makes itpossible to control the amount of molten resins flowing from the tips ofthe extruders through the resin distribution tubes to the multipleannular passage and this in turn makes it possible to vary the number ofkind of resins m, number of layers n, and number of sections p of themultilayer parison to be created without taking time for the change ofparts related to the die head and passages of resins and producemultilayer parisons with a variety of patterns required for hollowmolded articles.

Thus, the second mode of the process for multilayer blow molding andapparatus make it possible to bring together separately molten pluralthermoplastic resins inside the die head, extrude the molten resinsthrough a slit in the nozzle located at the tip of the die head tocreate a multilayer parison with a variety of patterns differing in thenumber of kind of resins m, number of layers n, and number of sectionsin the circumferential direction of the parison p, and mold the parisonto yield hollow articles having multiple sections of different kinds ofresins with each section meeting its respective property requirements.

Particularly desirable articles moldable by the second mode of theprocess and apparatus are chairs with their seats and/or backs moldedfrom thermoplastic resins and a variety of tables. More specifically,designating the number of resin layers in the face side as n_(f) andthat in the back side as n_(b), the relationship n_(f) ≠n_(b) holds andthe exposed outermost resin layer has two sections constituting the faceand the back. Each section here is formed by different resins, n_(f)being preferably 3 and n_(b) apparently 1.

It is desirable in the case of the seat and/or the back of a chair that,of the three resin layers on the face side, the outer layer in directcontact with the human body is formed by a flexible resin with aflexural modulus of 5,000 kgf/cm² or less and a Shore hardness A of 80or less, the middle layer by an adhesive resin, and the inner layer by ahard resin with a flexural modulus of 10,000 kgf/cm² or more and aRockwell hardness of 60 or more. It is also desirable that the resinlayer on the back side is formed by the same hard resin as in the innerlayer on the face side. As for the three resin layers on the face sideof a table, the exposed outer layer is formed by ABS resin, the middlelayer by an adhesive resin, and the inner layer by an olefinicelastomer. On the other hand, the resin layer in the back side of thetable is formed by a resin in the same category of the resin forming theinner layer of the face side. Of the chairs and tables molded fromthermoplastic resins in this manner, particularly relevant examples arechairs having a small table with collapsible frames on their backsuitable for installation in electric trains, aircraft, and theaters.

The third mode of the process of this invention relates to multilayerblow molding which extrudes plural thermoplastic resins from a die headin a tubular form to create a multilayer parison with a substantiallyuniform wall thickness in every part, introduces said parison into amold which is split into plural parts and open to receive said parison,and clamps said mold to perform blow molding which comprises formingsections different from one another in respect to one or more of thekind of resins, number of layers, and thickness of layers in thecircumferential direction of said parison and at the same time varyingthe wall thickness ratio of the resin layers in each of said sections inthe direction of the extrusion of said parison.

A multilayer parison extruded by this mode of blow molding has twosections and at least two resin layers, outer and inner, varying in thewall thickness ratio in the direction of the extrusion of the parison.Designating the ratio of the wall thickness of the inner layer to thatof the outer layer in one section as r₁ and the ratio in the othersection as r₂ in any one cross section in the longitudinal direction ofthe parison, it is recommended to perform the extrusion in such a manneras to maintain the product of r₁ and r₂ approximately unity.

An apparatus to practice the third mode of the process is a modificationof the apparatus for the aforesaid second mode of the process and isconstructed by forming two equal sections (p=2), each with acircumferential width of 180°, in the multiple annular passage, dividingsaid multiple annular passage into an array of passages and a resinjoint for combining the resins flowing through the the array ofpassages, and installing a ring-shaped passage width control valvebeneath each passage, said control valve being capable of moving backand forth in the direction at a right angle to that of the flow ofmolten resins and changing the width of the lower end of each passagewhereby changing the wall thickness ratio of the resin layers in eachsection of the parison in the direction of extrusion.

Hence, a multilayer blow molding apparatus thus constructed makes itpossible to extrude plural thermoplastic resins through a die head in atubular form to create a multilayer parison with a substantially uniformwall thickness in every part and blow mold said parison into a hollowarticle in which there are sections different from one another inrespect to one or more of the kind of resins, number of layers, andthickness of layers along the circumference in a cross section of themolded article, the wall thickness ratio of the resin layers in each ofsaid sections varies along the longitudinal wall of the molded article,and the changes in the thickness ratio are designed to occur to satisfythe performance requirements of each section of the molded article.

The third mode of the multilayer blow molding process and the apparatusmake it possible not only to extrude multilayer parisons with variouspatterns of m resins, n layers, and 2 circumferential sections and moldmultilayer hollow articles, but also to change the ratio of the wallthickness of the innermost layer to that of the outermost layer in eachsection along the longitudinal wall of the parison by actuating thepassage width control valves installed in the die head and mold hollowarticles with their properties changing markedly from part to part. Insimultaneous blow molding of plural hollow articles from one multilayerparison with vertical installation of plural cavities in a mold, thiscapability of changing the wall thickness ratio along the longitudinalwall allows installation of cavities alternately facing the oppositedirection and this arrangement of the cavities in turn distributes theclamping force uniformly throughout the mold at the time of clamping andimproves markedly the operating efficiency of blow molding.

The fourth mode of the process of this invention relates to multilayerblow molding which extrudes plural thermoplastic resins from a die headin a tubular form to create a multilayer parison with a substantiallyuniform wall thickness in every part, introduces said parison into amold which is split into plural parts and open to receive said parison,and clamps said mold to perform blow molding which comprises formingsections different from one another in respect to one or more of thekind of resins, number of layers, and thickness of layers in thecircumferential direction of the parison in at least one of resin layersconstituting said multilayer parison and at the same time varying thewidth of each section in the direction of the extrusion of the parison.

In the molding of multilayer parisons of m resins, n layers, and psections by this blow molding process, it is desirable to practice theprocess in such a manner as to produce different patterns of change inthe width of each section in the direction of the extrusion of theparison in any one of the resin layers. The creation of multilayerparisons in this manner has an advantage in that it allows selection ofproper kinds of resins to meet the performance requirements in the caseswhere the intended molded articles are polyhedrons of curved surfaceswith edges forming complex curves and each surface of the articles musthave properties of its own.

An apparatus to practice the fourth mode of the process is amodification of the apparatus for the aforesaid second mode of theprocess and is constructed by dividing the multiple annular passage intoan array of passages and a resin joint for combining plural resinsflowing through the array of passages and installing sectional widthcontrollers between the two, said sectional width controllers beingcomposed of funnel-shaped hemispherical shells in the numbercorresponding to that of resin layers n and tongue-shaped flapping nailsattached to the outer surface of each hemispherical shell near the lowerend of each partition wall constituting the sectional boundary in theaforesaid multiple annular passage and functioning to change the widthof each section and form a multilayer parison changing in the sectionalwidth in the direction of the extrusion of the parsion.

The sectional width controller to be installed between the array ofpassages and the resin joint may be formed integrally with one of thetwo or separately from the two.

The multilayer blow molding apparatus thus constructed makes it possibleto extrude plural thermoplastic resins through a die head in a tubularform to create a multilayer parison with a substantially uniform wallthickness in every part and blow mold said parison into a hollow articlein which there are sections different from one another in respect to oneor more variables of the kind of resins, number of layers, and thicknessof layers in the cross section of the molded article along thecircumferential wall, the width of each section in each resin layerchanges along the longitudinal wall of the molded article, and eachsection meets its respective performance requirements.

As with the first mode of the process and apparatus, the fourth mode ofthe process and apparatus make it possible not only to extrudemultilayer parisons with a variety of patterns of m resins, n layers,and p sections, but also to cause changes in the sectional width alongthe longitudinal wall of the parison with the aid of the sectional widthcontrollers installed in the die head and blow mold hollow articles withpatterns of marked changes in properties in both circumferential andlongitudinal directions, which allows respective sections of hollowarticles to perform sufficiently in heat resistance, water resistance,thermal insulation, sound insulation, and abrasion resistance inparticular end uses and contributes to improve the product performanceof the hollow articles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the cross section of a multilayerparison obtained in Example 1 by the first mode of the multilayer blowmolding process and apparatus of this invention.

FIG. 2 is an illustration in section of a multilayer blow moldingapparatus for the practice of the first mode of the process of thisinvention.

FIG. 3 is an illustration of the singled-out flow of resins inside themolding apparatus of FIG. 2.

FIGS. 4 and 5 are respective cross sections along the lines IV--IV andV--V of the lotus root of FIG. 2.

FIG. 6 is a schematic illustration of the cross section of a boiler-roomdoor blow-molded by the apparatus of FIG. 2.

FIG. 7 is a schematic illustration of the cross section of a multilayerparison extruded from the apparatus of FIG. 2.

FIGS. 8 and 9 are illustrations of a four-part mold being clamped in theblow molding of a multilayer parison.

FIG. 10 is a schematic illustration of the multilayer blow moldingapparatus of FIG. 2 to which blind rings are attached.

FIG. 11 is a perspective view of the blind ring of FIG. 10.

FIG. 12 is a plan view of a modification of the lotus root.

FIG. 13 is a schematic illustration of the cross section of a multilayerparison formed by the lotus root of FIG. 12.

FIGS. 14 to 17 are schematic illustrations of the sections of automotivebumpers formed by the multilayer blow molding apparatus of Example 1.

FIG. 18 is a schematic illustration in section of a multilayer blowmolding apparatus for the practice of the second mode of the process ofthis invention used in Example 2.

FIG. 19 is a perspective view of a passage-forming tube of FIG. 18.

FIG. 20 is an illustration of the singled-out flow of resins inside themolding apparatus of FIG. 18.

FIGS. 21 to 23 illustrate the flow of molten resins inside the resindistribution tubes equipped with branch-off points and junctions.

FIGS. 24 to 26 are schematic illustrations of the cross sections ofmultilayer parisons created by the process and apparatus of Example 2.

FIG. 27 is a perspective view of the seat and the back of a chair moldedfrom the multilayer parison of Example 2.

FIG. 28 is a schematic illustration of a section of the seat or back ofthe chair of FIG. 27.

FIG. 29 is a perspective view of a multilayer parison warping inwrinkles towards the side of the smaller resin flow while beingextruded.

FIG. 30 is a schematic illustration in section of a multilayer blowmolding apparatus to practice the third mode of the process of thisinvention relating to Example 3.

FIG. 31 is a view illustrating the ring-shaped passage width controlvalves of FIG. 30 and their vicinity.

FIG. 32 is a perspective view of the ring-shaped passage width controlvalve of FIG. 30.

FIGS. 33 and 34 illustrate how the ring-shaped passage width controlvalve functions.

FIG. 35 is a perspective view of a multilayer parison created by theprocess and apparatus of Example 3.

FIG. 36 is an illustration of cross sections of a multilayer parisoncreated by the process and apparatus of Example 3.

FIG. 37 illustrates a multilayer parison created by the process andapparatus of Example 3 being clamped in a mold.

FIG. 38 is an illustration in section of a multilayer blow moldingapparatus to practice the fourth process of this invention relating toExample 4.

FIGS. 39 to 42 illustrate how a flapping nail is attached.

FIG. 43 is a perspective view illustrating how passage-forming tubes anda shell are assembled.

FIGS. 44 and 45 are illustrations of the multiple annular passage inExample 4.

FIG. 46 illustrates a multilayer parison to be created in Example 4.

FIG. 47 illustrates how flapping nails change the sectional width ineach section.

FIG. 48 is an illustration in general of a multilayer parison extrudedby the fourth mode of the process and apparatus.

FIGS. 49 to 52 are schematic illustrations of partial cross sections ofmultilayer parisons created by prior art processes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The multilayer blow molding process, apparatus, and hollow articles ofthis invention are described in detail with reference to theaccompanying drawings and examples.

EXAMPLE 1

This example relates to the first mode of the multilayer blow moldingprocesss of this invention, apparatus, and hollow articles. FIG. 1conceptually shows a multilayer parison P₁ created by the process andapparatus in cross section. The multilayer parison P₁ has anm-resin-n-layer-p-section structure containing m kinds of resins R₁, R₂,. . . , and R_(m), n layers L₁, L₂, . . . , and L_(n), and p sectionsS₁, S₂, . . . , and S_(p) in the circumferential direction of theparison. The cross section is divided into p sections in FIG. 1 and, ineach section, the maximum number of resin layers is n and the maximumnumber of combinations of resins is m which is equal to or less than n.The multilayer parison P₁ in FIG. 1 is shown to have a uniform thicknessin its cross section all around the circumference, but the thickness maybe changed in any one or more of the sections S₁, S₂, . . . , and S_(p)as needed. Also, the number of resin layers n is shown to vary fromsection to section in FIG. 1, but it is allowable to form the resinlayers L₁, L₂, . . . , and L_(n) in concentric circles around thecircumference of the parison and form a necessary number of sections S₁,S₂, . . . , and S_(p) in each of these resin layers.

It is necessary for all of the sections S₁, S₂, . . . , and S_(p) formedin the circumferential direction of the parison to have a widthcorresponding to a central angle of 45° or more in at least one of theresin layers L₁, L₂, . . . , and L_(n) and to extend in the longitudinaldirection of the parison. This advantageously prevents a certain sectionof said parison from going out wholly from the mold cavity duringclamping of the mold and becoming totally missing in the blow-moldedarticle.

A sectional view of an apparatus useful for the practice of the firstmode of the multilayer blow molding process of this invention ispresented in FIG. 2 and the flow of molten resins inside the apparatusis singled out and illustrated in FIG. 3.

The die head of this molding apparatus is composed of a multitorus 1which spreads out five kinds of molten resins A, B, C, D, and E extrudedfrom five extruders (not shown) through resin inlet tubes 1b into aconcentric annular passage 1a, a lotus root 2 which constitutes passages2a for forming sections of different kinds of resins in a multilayerparison in the circumferential direction of the parison, an octopus 3which constitutes passages 3a connecting the multitorus 1 and the lotusroot 2 and directs the molten resins A, B, C, D, and E earlier spreadout into the five-fold concentric annular passage 1a in the multitorusinto the passages 2a formed in a pattern of m resins, n layers, and psections by the lotus root 2, and a nozzle 4 which is located beneaththe lotus root 2 and possesses a slit 4a which extrudes into a tubularmultilayer parison P₁ the molten resins A, B, C, D, and E which havepassed through the passages 2a in the pattern of m resins, n layers, andp sections in the lotus root 2 and joined beneath the passages 2a.

Referring to FIGS. 4 and 5 which are cross sections along the linesIV--IV and V--V of FIG. 2 (the cross sections of the lotus root in FIG.2 are those along the line II--II in FIGS. 4 and 5), the aforesaid lotusroot 2 in Example 1 has a plurality of passages 2a in the form of acircular arc or a slit for the molten resins and there are provided twokinds of bridges, 2b and 2c, which serve to connect the ends of eachpassage 2a lying on the same circumference. The bridge 2b is providedfor the purpose of mechanically connecting various circular arc- andslit-shaped passages 2a in the lotus root 2 to prevent them frombecoming loose. In order to minimize the traces of disturbance of theflow of resins by the bridge 2b in the molded products, the bridge 2bmust be made as narrow and short as possible. Thus, the bridge 2b is nolonger present in FIG. 5 which shows the cross section along V--V inFIG. 2. On the other hand, the other bridge 2c is provided specificallyto prevent the flow of resins. For example, the bridge 2c is provided inthe passages 2a for the resins B, C, and D in FIG. 2 and it extends in acircular arc as shown in broken lines in FIG. 4 and also extends roughlyalong the whole length of the lotus root 2 as shown in FIG. 2 in orderto prevent the flow of resins with certainty.

The part numbered 29 in FIG. 2 is a parison controller shaft driven by ahydraulic cylinder 30 and its up-and-down movement causes the die core4b of the nozzle 4 to move up and down and control the die slit 4athrough which a multilayer parison is extruded.

In Example 1, the multitorus 1, lotus root 2, octopus 3, and nozzle 4constituting the aforesaid die head H are provided as independent partswhich can be disassembled and reassembled and they are put together bymeans of a bolt 5a which connects the multitorus 1 and the lotus root 2with the octopus 3 inserted between the two and a bolt 5b which connectsthe nozzle 4 and the lotus root 2. With the multilayer blow moldingapparatus of Example 1 on hand, it becomes possible to keep a stock of avariety of shapes for the lotus root 2 and occasionally the octopus 3,select a suitable lotus root and occasionally an octopus required toform a multilayer parison P₁ of a desired m-resin-n-layer-p-sectionstructure, incorporate them in the die head H, and create a multilayerparison P₁ which can satisfactorily meet various customer needs forhollow molded articles.

A trial was made in Example 1 to blow mold a multilayer article G₁ whichhas a structure shown in FIG. 6 and can function as a heat- andsound-insulating boiler-room door.

Glass fiber-reinforced polypropylene, extremely easy to blow mold withstregnths at relatively high levels and at the same time economical, wasused as resin A for the structural material of the door. Glassfiber-reinforced polyphenylene sulfide with excellent resistance toheat, hot water, chemicals, and oils was used as resin B for the innersurface of the door which is directly exposed to an atmospherecontaining high-temperature steam, water vapor, and oil drops.Acrylonitrile-butadiene-styrene copolymer (ABS) with a smooth,beuatiful, glossy surface was used as resin C to form the outer surfacewhich is exposed to human eyes. Polyoxymethylene with good abrasionresistance and surface lubricity was used as resin D to form upper andlower ends of the door which are placed in contact with door rails. Inaddition, adhesive resin E was inserted between two resins which aredifficult to adhere to each other by themselves. These resins were fedby extruders (not shown) and put through the die head H in FIG. 2 tocreate a multilayer parison P, which is shown in cross section in FIG.7. The parison in question has five kinds of resins (A, B, C, D, and E),three layers (inner, middle, and outer), and four sections in the outerlayer (two sections of resin D each with a circumferential width of 45°,one section of resin B with a circumferential width of 135°, and onesection of resin C with a circumferential width of 135°).

The multilayer parison P₁ was then blown in the usual manner in a moldplaced immediately below the die head H to produce a multilayer hollowarticle G₁ (boiler-room door). The mold 6 used here is divided into fourparts, two 6a's, 6b, and 6c, corresponding to the four sections ofdifferent kinds of resins as shown in FIGS. 8 and 9. When the mold withthese four parts is clamped to perform blow molding, the areas 7 wherethe mold parts 6a, 6b, and 6c meet together pinch off the boundaries 8of the aforesaid sections of different kinds of resins and match theboundaries of the remaining sections along the angular edges of themolded article. In the blow-molded multilayer article G₁ thus produced,the two sections of resin D forming the upper and lower edges Occupy atleast 1/8 of the total length of the walls in cross section. The use offour-part mold 6 here has the following advantage. In blow molding witha conventional two-part mold, the concave parts on both sides of theupper edge in the cross section of a molded article lead to theformation of undercuts to interfere with the opening of the mold parts.On the other hand, the four-part mold 6 shown in FIGS. 8 and 9 canprevent the formation of such undercuts.

The multilayer blow molding apparatus shown in FIG. 2 fitted with thedie head H designed to extrude a multilayer parison P₁ of m resins, nlayers (n≧3), and p sections can be utilized to create a multilayerparison P₁ of m' resins, n' layers, and p' sections (m', n', p' arerespectively an integer of 2 or more and m≧m', n>n', and p≧p') byblocking the passages for the outer (n-n') layers out of the passages 2afor the whole n layers with the attachment of detachable blind rings 9to the lower openings of said passages for (n-n') layers in the lotusroot 2 as shown in FIG. 10. The blind ring is shaped like the one shownin FIG. 11, but not limited thereto. Any blind ring is satisfactory aslong as it can block the lower openings of the passages corresponding tothe outer (n-n') layers in the patterned passages 2a and it may beshaped like a circle or a circular arc. A die head H initially composedof multitorus 1, lotus root 2, and octopus 3 to extrude an n-layerparison P₁ can readily create a multilayer parison P₁ of less-than-nlayers with the attachment of a blind ring 9. This allows theconstruction of a multipurpose multilayer blow molding apparatus.

A modification of the lotus root 2 is shown in FIG. 12. There are threepassages 2a shaped like concentric semicircles on each side of thecentral line 0. It is possible to extrude a multilayer parison P₁ shownin FIG. 13 with 5 resins, 3 layers, and 2 sections (each with acircumferential width of 180°) by replacing the aforesaid lotus root inFIG. 2 with the modification of FIG. 12 and feeding five kinds of moltenresins. This modified lotus root can also create with ease a multilayerparison having three layers of three kinds of resins in one half and anapparent single layer of any one of the three kinds of resins in theother half or a multilayer parison of a half-multiple half-single layerstructure. In this case, the same resin is fed to all three passages 2afor the formation of the apparent single-layer section to assure theuniform thickness of the multilayer parison in whole.

In Example 1, the passages 3a of the octopus 3 connect the concentricannular passage 1a and the lotus root 2 in good order, inside to insideand outside to outside, as shown in FIG. 3. However, there is norestriction to the way these passages are arranged. For example, moltenresin C flowing through the outermost passage 1a may be led to theinnermost passage 2a by making proper connection in the octopus 3. Asfor the octopus 3, five kinds of resins introduced into the multitorus 1are all led to the patterned passages 2a in the lotus root 2. There isno limitation either here. It is allowable to block some of the passages1a in the multitorus 1, provide passages 3a to connect only theremaining passages 1a with the patterned passages 2a in the lotus root2, and distribute m kinds of molten resins (m being smaller now) tosections S₁, S₂, . . . , and S_(p).

The multilayer blow molding apparatus of Example 1 was used to produceautomotive bumpers G₂ having a variety of cross sections as shown inFIGS. 14 to 17.

The automotive bumper G₂ in FIG. 14 is composed of a beam 10 which is astructural material with good strength and toughness in the form of ahollow rib to absorb the shock of a collision and a fascia 11 which islaminated over the whole front of the beam 10 in direct view of humaneyes. In this example, the beam 10 is molded by glass fiber-reinforcedpolypropylene and the fascia 11 by ABS resin. The automotive bumper G₂in FIG. 15 has an adhesive resin layer 12 between the beam 10 and thefascia 11 for stronger bonding of the two parts and is blow-molded froma multilayer parison of a 3-resin-3-layer-2-section pattern. Theautomotive bumper G₂ shown in FIG. 16 has a resin layer 13 composed ofrecycled resin inside the beam 10 and is designed for effective reuse ofrecycled resins. The automotive bumper G₂ in FIG. 17 has the samestructure as the one in FIG. 16 with addition of an adhesive resin layer12 and can be molded from a multilayer parison of a4-resin-4-layer-2-section pattern.

The multilayer blow molding of these automotive bumpers G₂ can markedlyreduce the production cost and, in addition, minimize the thickness ofexpensive resins of good appearance in the visible fascia 11 in anintegral molding operation with the resultant marked reduction in therequirement of the expensive resins and in the product cost. Anotherindustrial merit is a realization of the use of recycled resins insidethe beam 10 and other unexposed parts.

EXAMPLE 2

Example 2 relates to the second mode of the multilayer blow moldingprocess, an apparatus, and hollow articles of this invention. Themultilayer blow molding apparatus to practice the second mode of theprocess is basically as illustrated in FIGS. 18 and 19. In the extrusionof a multilayer parison P₂, let m designate the number of kinds ofresins (m is an integer and ≧2), n the number of layers (n is an integerof 1 or more and is 2 or more in one or more sections), and p the numberof sections (p is an integer and ≧2) formed in the circumferentialdirection and different from one another in respect to one or more ofthe kind of resins, number of layers, and thickness of layers. The diehead H is composed of a multiple annular passage 20 which is constructedby putting an array of passage-forming tubes in the number correspondingto that of layers n or tubes 20a, 20b, and 20c in this example where n=3and a shell 20d together and a nozzle 21 which is attached to the lowerend of the multiple annular passage 20 and extrudes a multilayer parisonP₂. Slots 23 provided on the outer walls of the passage-forming tubes20a, 20b, and 20c in the number corresponding to that of sections p (p=2and the circumferential width is 180° in Example 2) form passages 22a,22b, and 22c for the molten resins in conjunction with the inner wallsof their respective outer neighbors 20b, 20c, and the shell 20d. Thetips of extruders (not shown) in the number corresponding at least tothe number of kind of resins m are connected to the upper ends of thepassages 22a, 22b, and 22c in the aforesaid multiple annular passage 20by means of the resin distribution tubes 24a, 24b, 24c, 24d, 24e, and24f. The flow of the molten resins is singled out and shown in FIG. 20.

In Example 2 where the number of layers is 3 and the number of sectionsis 2, it is possible to supply a maximum of 6 kinds of resins A, B, C,D, E, and F to form two equal sections with a circumferential width of180°. The resin distribution tubes 24a, 24b, 24c, 24d, 24e, and 24fattached to the tips of extruders respectively have one branch-off point25 and one junction 26 and each of them has a flow switchover valve 27as shown in FIGS. 21 and 22. With the aid of the branch-off points 25and the junctions 26 provided in the resin distribution tubes 24a, 24b,24c, 24d, 24e, and 24f, it is possible to control, for example, themolten resins to be supplied to two passages 22b in the multiple annularpassage 20 from the tips of extruders through the resin distributiontubes 24c and 24d as shown in FIG. 21. FIG. 22 illustrates how two kindsof resins C and D can be supplied to the opposite side of the passage 2bby changing the direction of flow with the aid of the flow switchovervalve 27. On the other hand, FIG. 23 illustrates how resin C is suppliedonly through the resin distribution tube 24c; the flow of the resin isdivided into the right and left direction by the branch-off point 25 andresin C is supplied through the junctions 26 to the whole of twopassages 22b. With the adoption of the resin distribution tubes 24a,24b, 24c, 24d, 24e, and 24f fitted with the branch-off points 25 and thejunctions 26, it becomes possible to combine resins C and D in fourdifferent ways, namely (C, D), (D,C), (C, C), and (D, D), in the passage22b formed by a passage-forming tube 20b by manipulating the flowswitchover valve 27. Since the same holds true to all the passages 22a,22b, and 22c, the number of patterns of layers in the multilayer parisonP₂ will be enormous even when not all of six kinds of resins are used.It is therefore possible to mold hollow articles of desirablespecification by selecting the number and kind of resins to meet theperformance requirements, selecting the most suitable pattern for thelayer structure of the multilayer parison to be created, and setting theflow switchover valves 27. There is no need here for changing partsinside the die head H.

The part numbered 28 in FIG. 18 is a bolt to fix the passage-formingtubes 20a, 20b, and 20c to the shell 20d and the part numbered 29 is aparison controller shaft to be driven by a hydraulic cylinder 30. Theup-and-down movement of the parison controller shaft 29 causes a similarmovement of a die core 21a of the nozzle 21 and controls the gap of thedie through which a multilayer parison is extruded.

If the multilayer parison to be created by the process and apparatus ofExample 2 were to have a structure of 6 kinds of resins (R₁ -R₆, resinsA-F), 3 layers (L₁ -L₃), and 2 sections (S₁, S₂), its cross sectionwould appear something like the one illustrated in FIG. 24. It ispossible to change the number of layers n from 1 to 3 freely andindependently in each of the two sections S₁ and S₂.

Moreover, the process and apparatus of Example 2 make it possible tocreate a multilayer parison P₂ in which the section S₁ is composed ofonly one kind of resin A in an apparent single layer and the section S₂is composed of three kinds of resins A, B, and C in three layers asshown in FIG. 25. A multilayer parison P₂ of such a structure issuitable for the molding of the seat G₃ and the back G₄ of a chair asillustrated in FIGS. 27 and 28. The face of the seat G₃ and that of theback G₄ coming into direct contact with the human body are composed ofthe three-layer section S₂ having resins A, B, and C. The resin Bforming the outer layer is a flexible resin which is soft, agreeable tothe touch, and not slippery and has a flexural modulus of 5,000 kgf/cm²or less and a Shore hardness A of 80 or less, for example, an elastomer.The resin A in the inner layer is a hard resin of excellent strength andtoughness with a flexural strength of 10,000 kgf/cm² or more and aRockwell hardness of 60 or more, for example, polypropylene. The resin Cin the middle layer is adhesive and bonds the outer layer of resin B andthe inner layer of resin A. The resin A in the back side must supportthe weight of a person sitting on it and is formed by the same hardresin as in the inner layer of the face side, for example,polypropylene.

The experiments conducted by the present inventors indicate that, in amultilayer parison P₂ such as shown in FIG. 25, it is advisable tocontrol the combined quantity of molten resins flowing into themultilayer section S₂ at 0.7 to 1.3 times, preferably 0.8 to 1.2 times,more preferably 0.9 to 1.1 times, that flowing into the apparentsingle-layer section S₁. If the flows of molten resins lose balance andtheir ratio goes out of the aforesaid range for some reason, forexample, by feeding the resin A into the single-layer section S₁ onlythrough one passage and thereby forming literally a single layer asillustrated in FIG. 26, the multilayer parison P₂ extruded from thenozzle 21 of the die head H shrinks in wrinkles towards the side of asmaller flow of resins (towards the single-layer section S₁ in thiscase) as illustrated in FIG. 29 and the molding operation sometimesbecomes impossible to perform because of excessive curving. A propercontrol of the combined flow of molten resins into the multilayersection S₂ and the flow of molten resins into the apparent single-layersection S₁ can realize a substantially uniform wall thickness in everypart regardless of the number of layers n in each section.

EXAMPLE 3

This example relates to the third mode of the multilayer blow moldingprocess, an apparatus, and hollow articles of this invention.

The multilayer blow molding apparatus to practice the third mode of theprocess is, as shown in FIGS. 30 and 31, basically the same as that forthe practice of the second mode shown in FIG. 18 and has the followingadditional features. In the die head H, two equal sections (p=2) with asectional width of 180° are formed in the multiple tubular passage 20 asin the aforesaid Example 2. The multiple annular passage 20 is dividedinto a passage 20_(x) which contains passages 22a, 22b, and 22cconstructed by putting three tubes 20e, 20f, and 20g and a shell 20htogether and a resin joint 20y where the plural resins flowing through20x join. A ring-shaped passage width control valve 31 is installedbetween 20x and 20y beneath each passage 22a, 22b, or 22c and the saidvalve 31 is capable of making a reciprocating motion in the direction ata right angle to the flow of molten resins and changing the width of thelower opening of each passage 22a, 22b, and 22c and, in turn, changingthe wall thickness of the resin layer in each section in the directionof the extrusion of the parison.

The parts numbered 21, 28, 29 and 30 in FIGS. 30 and 31 referrespectively to a bolt used for assembling the nozzle, threepassage-forming tubes 20e, 20f, and 20g, and the shell 20h, a parisoncontroller shaft, and a hydraulic cylinder as in the above-mentionedExample 2.

The passasge width control valve 31 in this Example 3 has two rings 31aand 31c which are arranged concentrically with a given distancemaintained between them and linked together at two sites as illustratedin FIG. 32. The ring 31a or 31c has a bevel 33 which contacts a seat 32provided at the lower edge of the tube 20f or 20g. The passage widthcontrol valve 31 is connected to a pair of hydraulic cylinders 34provided on both sides in the direction of motion and is made to movetowards right or left by the hydraulic cylinders 34 as illustrated inFIGS. 33 and 34. A motion of the said valve to one side decreases thewidth of the lower opening of one of the passages 22a and 22c in onesection of 20x and at the same time increases that in the other sectionon the opposite side. This motion also increases the width of the loweropening of the other of the passages 22a and 22c in one section anddecreases that in the other section on the opposite side. In FIG. 33,the passage width control valve 31 has shifted to the end of its stroketowards the section S₁ to maximize the wall thickness of the outer layer35a in the section S₁ and that of the inner layer 35b of the section S₂and minimize the wall thickness of the inner layer 35b in the section S₁and that of the outer layer 36a in the section S₂. Contrarily, in FIG.34, the passage width control valve 31 has shifted to the end of itsstroke towards the section S₂ to minimize the wall thickness of theouter layer 35a in the section S₁ and that of the inner layer 36b in thesection S₂ and maximize the wall thickness of the inner layer 35b of thesection S₁ and that of the outer layer 36a in the section S₂. In FIGS.33 and 34, the part numbered 38 is a piping to drive the aforesaid pairof hydraulic cylinders 34 and the symbol 40 is a vector indicating thedirection of motion and the width of motion of the passage width controlvalve 31.

The multilayer parison P₃ to be created in this Example 3 has a3-resin-3-layer-2-section structure as shown in FIG. 35. In its twosections S₁ and S₂, the wall thickness of the outer layers 35a and 36aand that of the inner layers 35b and 36b change in the direction of theextrusion of the parison, differently in the upper region 37a and thelower region 37b. Designating the wall thickness of the inner layer 35band that of the outer layer 35a in the section S₁ respectively as t₁ andt₂ and their ratio as r₁ and likewise t_(1'), t_(2'), and r₂ in respectto the section S₂ in a cross section at any point in the longitudinaldirection of the parison P₃, r₁ and r₂ are seen to change in such amanner as to keep the product of r₁ and r₂ approximately unity.

In consequence, the process and apparatus of Example 3 make it possiblenot only to extrude a multilayer parison P₃ of anm-resin-n-layer-2-section pattern and mold a variety of hollow articlesas in Examples 1 and 2 but also to vary the wall thickness ratio of theinner and outer layers in each of the sections S₁ and S₂ in thelongitudinal direction of the parison with the aid of the passage widthcontrol valve installed inside the die head and to mold hollow articlesexhibiting a pattern of marked property changes from region to region.This capability of changing the wall thickness ratio in the longitudinaldirection of the parison offers the following advantage in thesimultaneous blow molding of plural hollow articles from a singlemultilayer parison with a vertical arrangement of plural cavities; itnow becomes possible to arrange each cavity alternately facing theopposite direction, distribute the clamping force uniformly over themold, and improve the operating efficiency of blow molding markedly.

The case in point is the molding of the seat and back of a chair earlierillustrated in FIGS. 27 and 28 in Example 2. As shown in FIGS. 36 and37, the multilayer parison P₃ extruded from the nozzle 21 of the diehead H has an exactly opposite relationship in the wall thickness ratioof the inner and outer layers in its cross section in the upper region37a and the lower region 37b of the two sections. Also, the upper cavity39a and the lower cavity 39b of the mold 39 are positioned facing theopposite direction and this allows the clamping force to act uniformlyall over the mold 39 when the mold halves are clamped to perform blowmolding.

EXAMPLE 4

This example relates to the fourth mode of the multilayer blow moldingprocess, an apparatus, and hollow articles of this invention.

The apparatus to practice the fourth mode of the process is illustratedin FIGS. 38 and 39 and is basically the same as the one to practice thesecond mode of the process shown in FIG. 18 with the following featuresadded. The multiple annular passage 20 in the die head H is divided intoa passage 20_(x) which contains annular passages 22a, 22b, and 22cconstructed by putting three tubes 20i, 20j, and 20k and a shell 20mtogether and a resin joint 20y where plural resins flowing through 20xjoin. A passage width controller 41 is installed as an integral partbetween 20x and 20y beneath 20x to change the width of each section inthe circumferential direction of the parison for each resin layer. Thepassage width controller 41 is composed of funnel-shaped hemisphericalshells 42a, 42b, 42c, and 42d formed integrally at the lower ends of thepassage-forming tubes 20i, 20j, and 20k and the shell 20m andtongue-shaped flapping nails 43a and 43c which are attached to the outersurface of the hemispherical shells 42a and 42c at a locationcorresponding to the lower end of the partition wall constituting theboundary of each section in the passage 20x and oscillate to change thewidth of each section of the parison to be created. This device enablesthe creation of multilayer parisons with their sectional width changingin the direction of the extrusion of the parison.

The positional relationship of the hemispherical shells 42a, 42b, 42c,and 42d and the oscillatory tongue-shaped flapping nails 43a and 42cconstituting the sectional width controller 41 in Example 4 isschematically shown in FIG. 39 in respect to the outermost hemisphericalshell 42d, the hemispherical shell 42c which is the inside neighbor of42d, and the flapping nail 43c. The flapping nails 43a and 43c areconstructed as shown in FIGS. 40 to 42 with 43c taken as an example. Theflapping nail 43c is provided with a driving shaft 45 which is connectedto the driving device 44 (shown in FIG. 38). The driving shaft 45 iscomposed of a torque shaft 45a which transmits a rotating motion to theflapping nail 43c, a support pipe 45b which is tubular and threaded atits tip into the flapping nail 43c, and a pin 45c which holds the torqueshaft 45a and the support pipe 45b together. The upper and lowersurfaces of the flapping nails 43a and 43c are curved so that they fitthe hemispherical shells 42b and 42d on the outside and thehemispherical shells 42a and 42c on the inside.

Arch-like fittings 46i, 46j, 46k, and 46m are provided on the top of thepassage-forming tubes 20i, 20j, and 20k and the shell 20m constitutingthe multiple annular passage 20 as shown in FIG. 43 and a bolt 28penetrating through these fittings holds them together to secure aconcentric arrangement of 20i, 20j, 20k, and 20m. The shafts 30a of thehydraulic cylinders 30 attached to the shell 20m are fixed to the upperend of the parison controller shaft 29 running through the tube 20i. Thehydraulic cylinders 30 move the parison controller shaft up and down andthis motion, in turn, moves the die core 21 of the nozzle 21 up and downto control the gap of die through which the multilayer parison isextruded.

In Example 4, the two annular passages 22a and 22c shown in FIG. 38 aredivided into four sections S₁, S₂, S₃, and S₄ as shown in FIGS. 44 to 46and each of the flapping nails 43a and 43c is provided at a locationfollowing the lower end of the partition wall (not shown) dividing thepassages 22a and 22c into the four sections. There is no section formedin the remaining annular passage 22b. The resin distribution tube 24a isconnected to the two sections S₁ and S₃ formed in the annular passage22a to supply molten resin A from an extruder (not shown), the resindistribution tube 24b is connected to the other two sections S₂ and S₄formed in the annular passage 22a to supply molten resin B from anextruder (not shown), and the resin distribution tube 24c is connectedto the annular passage 22b to supply molten resin C. Moreover, the resindistribution tube 24d is connected to the two sections S₁ and S₃ formedin the annular passage 22c to supply molten resin D from an extruder(not shown) and the resin distribution tube 24e is connected to theother two sections S₂ and S₄ formed in the annular passage 22c to supplymolten resin E from an extruder (not shown). In FIG. 44, it is necessaryto feed resins A and B which form inner layers upstream of resin C whichforms a layer outside of A and B and also to feed resin C upstream ofresins D and E which form layers outside of C for the following reason.Suppose the resin distribution tubes 24a and 24b are placed downstreamof the resin distribution tube 24c, resins A and B flowing from thetubes 24a and 24b into the annular passage 22a located farthest withinthe multiple annular passage 20x must penetrate through resin C flowingdown the annular passage 22c located outside of the passage 22a.

In FIG. 47 is schematically shown the way to control the width of thesections S₁, S₂, S₃, and S₄ with reference to a planar development ofthe annular passage 22c. It is possible to vary the width of thesections S₁, S₂, S₃, and S₄ by oscillating the four flapping nails 43cattached to the lower ends of the partition walls 47 dividing thepassage 22c into the four sections at a specified timing in a specifiedwidth. The arrow 48 indicates the direction of flow of the molten resinsD and E.

In general, a multilayer parison P₄ extruded by the fourth mode of theprocess and apparatus has an m-resin-n-layer-p-section pattern (m is thenumber of kind of resins R, n is the number of layer of resins L, and pis the number of sections S in the circumferential direction) with thesectional width changing in the direction of the extrusion of theparison as illustrated in FIG. 48.

In Example 4, the partition walls dividing the annular passage 22a andthose dividing the annular passage 22c are aligned and the flappingnails 43a and 43c are positioned on the same line connecting thosepartition walls. This arrangement, however, is not absolutely necessary.The flapping nails 43a and 43b may be shifted towards right or left andthey may be controlled to flap independently so as to form sectionsshowing different patterns of change in their outer and inner layers.

The aforesaid die head H is assembled as follows.

1 Place the assembled multiple passage 20x upon the sectional widthcontroller 41 and fasten with a bolt 28 as shown in FIG. 38.

2 Insert the flapping nails 43a and 43c respectively between thefunnel-shaped hemispherical shells 42a and 42b and between funnel-shapedhemispherical shells 42c and 42d and hold them in place with the aid ofa jig.

3 Insert the torque shaft 45a from the outside of the outermostfunnel-shaped hemispherical shell 42d and fix it to each of the flappingnails 43a and 43c by inserting the square tip 49 of the torque shaftinto the square hole 50 of the flapping nail.

4 Insert the support pipe 45b over the torque shaft 45a and turn themale screw 51 at its tip into the female screw 52 in each of theflapping nails 43a and 43b.

5 Drive the pin 45c through the torque shaft 45a and the support pipe45b and put them together to construct the driving shaft 45 for each ofthe flapping nails 43a and 43c.

6 Attach the driving device 44 to the driving shaft 45 thus assembled.

7 After completion of the mounting of the flapping nails 43a and 43c,attach the resin joint 20y to the lower end of the unit of the multiplepassage 20x and the sectional width controller 41 assembled in 1.

8 Insert the parison controller shaft 29 into the unit of the multiplepassage 20x, the sectional width controller 41, and the resin joint 20yassembled in 7 from below along the central axis and connect its upperend to the shaft 30a of the hydraulic cylinder 30 installed above themultiple passage 20x.

9 Finally, attach the resin distribution tubes 24a, 24b, 24c, 24d, and24e to the multiple passage 20x at the specified positions.

Thus, the multilayer blow molding apparatus of Example 4 makes itpossible to extrude five kinds of thermoplastic resins in a tubular formfrom the die head H to create a multilayer parison P₄ with asubstantially uniform wall thickness in every part and mold said parisoninto a variety of hollow articles. These hollow articles have sections Sdifferent from one another in respect to one or more of the number ofkind of resins m, number of layers n, and thickness of layers along thecircumference in their cross section and varying in the width alongtheir longitudinal cross section. It is hence possible to blow moldhollow articles with a pattern of marked property change both in theirtransverse section and in their longitudinal section, draw out from theresins in various sections of the hollow articles sufficientperformances in such properties as heat resistance, water resistance,oil resistance, toughness, and abrasion resistance for the intended enduses, and improve the product performance of hollow articles.

What is claimed is:
 1. A multilayer blow molding process, comprisingextruding plural thermoplastic resins from a die head in a tubular formto create a multilayer parison having a substantially uniform wallthickness in every part, introducing said multilayer parison into a moldsplit into plural parts and open to receive said parison, clamping saidmold and conducting blow molding, said multilayer parison havingsections different from one another in respect to one or more of thekind of resins, number of layers, and thickness of layers in thecircumferential direction of said parison in at least one of resinslayers and varying the width of each section in the direction of theextrusion of parison by a passage width controller having tongue-shapedflapping nails which oscillate to change said width of each section ofsaid parison.
 2. A multilayer blow molding process of claim 1 wherein amultilayer parison of an m-resin-n-layer-p-section pattern is createdwherein the width of a layer is varied in the direction of the extrusionof said parison to thereby vary the width of each section.
 3. Amultilayer blow molding process, comprising extruding pluralthermoplastic resins from a die head in a tubular form to create aparison having a multilayer structure all around the circumference and asubstantially uniform wall thickness in every part, introducing saidmultilayer parison into a mold split into plural parts and open toreceive said parison, clamping said mold and conducting blow molding,the multilayer parison having sections different from one another inrespect to one or more of the kind of resins, number of layers, andthickness of layers in the circumferential direction of said multilayerparison, with a section of the smallest width of said sections ofdifferent kinds of resins in any one of the resin layers having acircumferential width of 45° or more and extending in the longitudinaldirection of said multilayer parison, wherein said die head comprises amultitorus which spreads plural molten resins emerging from extrudersinto concentric annular passages, a lotus root which constitutespassages for forming sections corresponding to different kinds ofresins, number of layers, and thickness of layers in the circumferentialdirection of the multilayer parison, an octopus which forms passagesconnecting said multitorus and said lotus root, and a nozzle which islocated beneath said lotus root and extrudes said multilayer parison. 4.A multilayer blow molding process, comprising extruding pluralthermoplastic resins from a die head in a tubular form to create amultilayer parison having a multilayer structure all around thecircumference and a substantially uniform wall thickness in every part,said multilayer parison having sections different from one another atleast in respect to the thickness of layers in a horizontalcross-section of said parison, introducing said multilayer parison intoa mold split into plural parts and open to receive said parison,clamping said mold and conducting blow molding, said die head havingplural passages, each passage delivering a thermoplastic resin through alower opening to a resin joint where the resins flow together, said diehead having a ring-shaped passage width control valve beneath saidpassages for varying the width of the lower opening of each passage, andmoving said ring-shaped width control valve in a direction perpendicularto the direction of extrusion to vary the width of the lower opening ofeach passage and thereby varying the wall thickness ratio of the resinlayers in each section in the direction of the extrusion of said parisonby said ring-shaped passage width control valve.